A method called an MS/MS analysis (also called a tandem analysis) is known as one of the mass spectrometric techniques for identification and structural analyses of compounds having large molecular weights. A tandem quadrupole mass spectrometer (also called a triple quadrupole mass spectrometer) having a relatively simple and inexpensive structure is one of the widely used mass spectrometers capable of the MS/MS analysis.
As disclosed in Patent Literature 1, generally in the tandem quadrupole mass spectrometer, quadrupole mass filters are respectively provided at the front and rear stages of a collision cell for dissociating ions so as to sandwich the collision cell. Precursor ions are selected by the front-stage quadrupole mass filter from among a variety of ions originating from a target compound, and product ions are separated by the rear-stage quadrupole mass filter in accordance with the mass-to-charge-ratio. The collision cell has a box-like, relatively tight-sealed structure, and a CID gas such as argon and nitrogen is introduced into the collision cell. The precursor ions selected by the front-stage quadrupole mass filter are introduced into the collision cell endowed with appropriate collision energy, and collide with the CID gas inside the collision cell. As a result, collision-induced dissociation occurs, and the product ions are produced.
The dissociation efficiency of ions inside the collision cell depends on the amount of collision energy of the ions, the CID gas pressure inside the collision cell, and the like. Hence, the detection sensitivity of the product ions that have passed through the rear-stage quadrupole mass filter also depends on the amount of collision energy and the CID gas pressure.
In the tandem quadrupole mass spectrometer, a measurement in a multiple reaction monitoring (MRM) mode is performed in many cases, in order to perform quantitative determination on a known compound with high accuracy. In the MRM measurement mode, for both the front-stage and rear-stage quadrupole mass filters, the mass-to-charge-ratios of the ions that pass through the filters are fixed. Hence, in conventional tandem quadrupole mass spectrometers, the CID gas pressure inside the collision cell is set to a value (normally, at several mTorr) in advance by a manufacturer such that the detection sensitivity is as high as possible in the MRM measurement mode.
In general, as the CID gas pressure inside the collision cell becomes higher, ions become more likely to contact the CID gas, and hence the dissociation efficiency of the ions becomes higher. However, the kinetic energy of the ions (both the precursor ions and the product ions) is attenuated by the collision with the gas, the flight speed of the ions decreases as a whole, and the variation range of the speed increases. In the case of the MRM measurement mode, dissociation of ions having the same mass-to-charge-ratio and selection and detection of product ions having the same mass-to-charge-ratio are performed for a certain amount of period, and hence the decrease in ion flight speed and the increase in speed variation range in the collision cell as described above have relatively small influences.
However, in the case of a precursor ion scan measurement mode, a neutral loss scan measurement mode, and the like in which the front-stage quadrupole mass filter performs a scan over a predetermined mass-to-charge-ratio range, the decrease in ion flight speed and the increase in speed variation range inside the collision cell may cause problems. That is, if the front-stage quadrupole mass filter performs a mass scan at a high speed, the following phenomenon is more likely to occur: before a product ion produced from a precursor ion having a given mass-to-charge-ratio M reaches a detector, a product ion produced from a precursor ion having a mass-to-charge-ratio M+ΔM catches up with the product ion. This means that separation between a given ion peak and another ion peak adjacent to the given ion peak on a mass spectrum (MS/MS spectrum) deteriorates. Moreover, if the range of the flight speed of the product ions increases, a decrease in peak top intensity becomes remarkable.